The invention relates to communications systems and methods, and more particularly to a data communications system and method in which channel impairments are treated based on at least a measure of a digital loss in transmitted signals imposed by a switch in a central office.
It is well known that a public switched telephone network (PSTN) comprising T1 facilities can form a basis for a virtual digital network providing 64 kb/s channels. For example, by synchronizing a pulse code modulation (PCM) modem to an 8 kHz sampling rate provided in a central office and using 8-bit PCM words for data transmission, the modem can theoretically achieve a data rate up to 64 kb/s.
However, in practice, due to power constraints and such channel impairments as echo and intersymbol interference, the highest data rate achievable by the PCM modem is about 56 kb/s. This rate may be further reduced as the central office periodically xe2x80x9crobsxe2x80x9d the least significant bit (LSB) of the PCM words and substitutes it with a signaling bit. As is well known, the robbed bit signaling is necessary for indicating call statuses to effect call administration in the PSTN.
To reduce echo interference in traditional voice communications, especially far echo interference due to a long-distance feedback of a voice signal through the PSTN, the level of the voice signal from the PSTN is attenuated in a central office switch before it is passed onto an analog loop connected to telephone equipment. Such attenuation by the central office switch is known as a xe2x80x9cdigital loss.xe2x80x9d
While the above robbed bit substitution does not cause significant distortion in voice communications, it causes significant degradation in data communications because of the loss of transmitted bits occasioned thereby. Similarly, while the above digital loss helps reduce the far echo interference in voice communications, it causes the levels of transmitted signals representing data to be attenuated, resulting in erroneous data recovery in data communications if the digital loss is not taken into account in the PCM modem.
Although the digital loss is built into each central office switch and the underlying attenuation factor is invariant as far as the switch is concerned, this factor may vary from one switch to another depending on its type and manufacturer. As a result, a PCM modem which is pre-adjusted during manufacture thereof to allow for the digital loss by a particular switch may not function properly when connected to a different switch in the field. In addition, an accurate measure of the actual digital loss in the field is needed in some PCM modems to properly update an echo canceller therein. Further, by knowing the actual amount of digital loss, the level of the signal to be transmitted may be boosted, in anticipation of the digital loss, to exploit the full range of the power level allowed by the analog loop, thereby increasing the signal to noise ratio of data transmission and thus accuracy of data recovery. However, a measurement of the digital loss is often ineffective as it is obscured by the robbed bit signaling, which affects the transmitted signal level from time to time.
Accordingly, there exists a need for, apart from properly treating the robbed bit signaling and the digital loss in data communications, a methodology for effectively measuring the digital loss, notwithstanding that such a measurement is obscured by the robbed bit signaling.
In accordance with the invention, a digital loss imposed by a central office switch is effectively measured using a signal level conversion table, which is created in a xe2x80x9clevel learningxe2x80x9d process, forming part of training of a PCM modem before its operation. This table contains (a) each allowable transmitted signals representative of data which are subject to robbed bit signaling and affected by the digital loss, and (b) the received signal level corresponding thereto. In addition, any transmitted signals which are actually affected by the robbed bit signaling, and the corresponding received signals, are identified in the level learning process.
It is well known that the digital loss may cause different signal levels of transmitted signals representing different data to be attenuated to virtually the same signal level. That is, transmitted signals having different signal levels may emerge from the central office switch with virtually the same signal level because of the digital loss, even though they represent different data. As a result, the received signal levels in the signal level conversion table corresponding to different transmitted signals may be repetitive. The amount of the digital loss is a function of the number of sets of repetitive received signal levels.
In accordance with an aspect of the invention, the received signal levels which correspond to those transmitted signals unaffected by the robbed bit signaling are selected from the signal level conversion table. One or more sets of repetitive signal levels are identified from the selected received signal levels. The amount of the digital loss is determined based on the number of sets identified.